The present invention relates generally to methods of manufacturing microwave waveguide components, and more particularly, to methods of manufacturing microwave waveguide components using molded, cold machined metallized plastic.
For microwave applications, waveguides and waveguide assemblies are generally fabricated from metal. The most commonly used metallic materials are aluminum alloys (alloy numbers 1100, 6061, and 6063 per ASTM B210 and cast brazable alloys such as 712.0, 40E, and D612 per QQ-A-601), magnesium alloy (alloy AZ31B per ASTM B107), copper alloys (per ASTM B372 and MIL-S-13282), silver alloy (grade C per MIL-S-13282), silver-lined copper alloy (grade C per MIL,-S-13282), and copper-clad invar. These materials may be divided into two classes - rigid and flexible. The rigid materials are either wrought, drawn, cast, electroformed, or extruded, while the flexible materials consist of convoluted tubing. If these materials are not formed to net shape, they are either machined to shape (when all features are accessible) or broken down into individual components and joined together to form complex assemblies. Additional information regarding rigid rectangular waveguides can be found in MIL-W-85G, while rigid straight, 90 degree step twist, and 45-, 60-, and 90 degree E and H plane bend and mitered comer waveguide parameters are given in MIL-W-3970C ASTM B102 covers magnesium alloy extruded bars, rods, shapes, and tubes. Aluminum alloy drawn seamless tubes and seamless copper and copper-alloy rectangular waveguide tubes are discussed in ASTM B210 and ASTM B372, respectively. Waveguide brazing methods are given in MIL-B-7883B, while electroforming is discussed in MIL-C-14550B. It is in the fabrication of complex shapes that the disadvantages of metallic waveguides become most apparent.
For complex structures where forming or machining the metal to net shape is not possible, machining into individual components (preferably by numerically controlled cutting tools) is employed. These components can then be joined using either brazing, bending, soldering, or electron beam welding. Brazing, as described in MIL-B-7883, can be performed using either dip, furnace (also called inert gas brazing), or torch techniques; vacuum brazing may also be employed. Dip brazing is comprised of submerging the components to be joined into a molten bath of salt or flux, followed by quenching them slowly in hot water to dissolve the salt or flux. Inert gas and vacuum brazing are fluxless, expensive techniques that are performed with the components fixtured prior to heating them, in vacuum in the presence of a filler metal. The filler metal melts, forming the braze joint. Torch brazing, used primarily for joint touch-up, involves preheating the parts with a neutral or slightly reducing flame in order to liquify the filler metal. This filler metal is introduced at one site on one of the mating sun:aces only; its flow path forms the braze joint.
All of the brazing methods have the following disadvantages. Measurable part distortion occurs, and in many cases, the amount of distortion is unacceptable in terms of the degradation of the microwave component's electrical performance. The thickness of the original joints to be brazed is reduced during the brazing operation. This material loss is not a controllable variable. The heat treatment of the brazed alloy is degraded. The brazing operation can cause latent defects in brazed hardware that are joined due to residual flux or poor quality filler metal. Residual flux can result in corrosion. The use of excessive flux or filler metal can result in excessively large fillets, which can be detrimental to the microwave component's electrical performance.
When metallic components are bonded, conductive adhesives are utilized. The conductive adhesives give inferior bond strengths compared to nonconductive structural adhesives used in joining plastic parts. In addition, use of the conductive adhesive in the metallic parts can result in radio frequency (RF) and physical leakage of the final assembly, causing poor electrical performance and potentially allowing fluid entrapment in the assembly.
When metallic components are soldered together, creeping of the metal at joint locations becomes a significant problem, and leads to joints that are not structurally sound. Electron beam welding is a costly and difficult to control process for joining metallic components, and involves the "coalescence of metals by the heat obtained from a concentrated beam of high velocity electrons impinging upon the surfaces to be joined" ("Welding Handbook," Seventh Edition, Volume 3, W. H. Kearns, American Welding Society, 1980). Weld quality control using electron beam welding is more problematic than adhesive bond line control due to inherent difficulties in controlling the angle of beam incidence, evacuation time penalties, and width-to-depth ratios of the weld itself.
Accordingly, it would be an advance in the art to have a process of fabricating microwave waveguide components that provides for less costly and more producible components that achieve performance levels comparable to conventional metal waveguide components.